The obvious answer, of course, was driver dancing...
I used to work with somebody who drank about twenty cans of coke a day. Full fat coke, you understand, with all of the badness that implies. But the effect of the caffeine in the coke was to leave her a mass of twitching and tics and fidgetiness and thereby to render her skinny as a rake. I've never been sure if it would work for me because I used to fidget a lot at my desk, to the extent that I could wear holes in carpets and yet I remained resolutely XXL.
But driver dancing seemed the way to go though it raised the issue of how legal it might be. As I understand it, the real problem is driving without due care and attention. So, I always kept my hands on the steering wheel and my eyes on the road and I only went absolutely crazy and busted out proper moves when I was actually at a complete standstill in traffic and not too visible to other road users.
My secondary calorie burning tactic was to sing at the top of my voice to all tunes, no matter what they were. Does singing burn calories? Well, sound waves require energy so I'm assuming that singing is more calorie-burning then sitting in a line of traffic fuming at the idiocy of fellow road users especially, I have to say, BMW drivers who win the award of 'Car Most Likely to Tailgate Me and to Be Issued with a Complimentary Finger'.
And that was my third calorie burning strategy. Every car that drove up my arse was an excuse to burn a minor amount of calories via the medium of raising my forearm and extending a digit. Also, possibly to be accompanied by the loud shouting of complimentary character assessment. Because shouting is almost as good as singing when it comes to expending energy.